Absorbers that are covered on one side thereof with a microperforated sheet towards a sound source are known. The absorption can be influenced by the perforation geometry, the distance between perforations and the open surface area of the sheet. In the automobile field, such structures are known specifically with a perforated metal foil and an absorber for the engine compartment.
DE 10 2004 050 649 A1 describes a sound-absorbing heat shield. A sound-absorbing heat shield with a sheet metal carrier plate which holds an acoustically and thermally effective insulation layer is disclosed. The heat shield comprises a protective sheet having microperforations and provided on the side of the insulation layer facing a sound source when in use.
A heat- and sound-insulating lining for the engine compartment of motor vehicles, which is described in DE 101 43 167 A1, consists of a heat reflector microperforated on the engine side, in contact with a polyurethane foam layer which is impregnated on the engine side with thermoset material having long-term temperature resistance of 200° C., especially of 150° C., for three weeks, in contact with a cover layer facing away from the engine side.
The production of microperforated sheets, especially metal foils, has long been known. For example, reference may be made to U.S. Pat. No. 7,838,125 B2, in which a specific embodiment of a microperforated metal foil is described. It is described that the metal foil is deformed by needle punching in such a way that a crater elevated with respect to the mean sheet layer level whose lower edge frays when the needle punctures through is formed against the needle puncture direction.
DE 10 2004 053 751 A1 describes a lining part for a vehicle, especially an underbody panel. It possesses a porous middle layer and at least one cover layer on each side, the porous middle layer having such a structure that it has acoustic transparency or acoustically absorbing efficiency. The acoustically absorbing porous middle layer may be covered on one or both sides thereof with one or more acoustically transparent or absorbing cover layers.
Structures with an absorber and a microperforated polymer sheet are known for the interior area of motor vehicles. EP 1 101 218 B1 describes a microperforated polymer sheet for use in a sound absorber, which is formed with a polymer sheet of a defined thickness and several microperforations, wherein the microperforations each have a narrowest diameter that is smaller than the sheet thickness, and a broadest diameter that is greater than 125% of the narrowest diameter.